part revert, part redo 'which' replacement

In e75e5879 'replace "which" with "command -v" for portability' I missed
that command -v isn't actually required to be available in debian, so
for the 5 files we are using it:

Two (abicheck/run_abi_test & test/integration/framework) are called in
environments were I believe sh is at least dash or 'better' as the first
one is "interactive" for apt developers and the later is sourced by ~200
tests in the same directory run by hand and ci-services – for the later
we have pulled some uglier hacks for worser things already, so if there
should actually end up needing something more compatible we will notice
eventually (and the later actually had a command -v call for some time
already and nobody came running).

debian/rules and debian/apt.cron.daily I switched back to which as that
is more or less debian-specific or at least highly non-critical.

That leaves cmdline/apt-key.in with a bunch of calls where I will
implement that functionality in shell as this is relatively short-lived
as it is used to detect wget (for net-update, which Michael wants to
revive and in that process will properly use apt-helper instead of wget)
and to detect gpg vs. gpg2 systems, where the earlier is supposed to go
away in the longrun (or the later, but by replacing the earlier…).
[and this gpg/gpg2 detection is new in sid, so I have some sympathy for
that being a problem now.]

Thanks: Jakub Wilk for pointing out #747320

cmdline/apt-key.in

@@ -25,6 +25,21 @@ requires_root() {
 	fi
 }
 
+command_available() {
+    # command -v "$1" >/dev/null 2>&1 # not required by policy, see #747320
+    # which "$1" >/dev/null 2>&1 # is in debianutils (essential) but not on non-debian systems
+    local OLDIFS="$IFS"
+    IFS=:
+    for p in $PATH; do
+	if [ -x "${p}/${1}" ]; then
+	    IFS="$OLDIFS"
+	    return 0
+	fi
+    done
+    IFS="$OLDIFS"
+    return 1
+}
+
 get_fingerprints_of_keyring() {
     aptkey_execute "$GPG_SH" --keyring "$1" --with-colons --fingerprint | while read publine; do
 	# search for a public key
@@ -112,7 +127,7 @@ net_update() {
     fi
     # in theory we would need to depend on wget for this, but this feature
     # isn't useable in debian anyway as we have no keyring uri nor a master key
-    if ! command -v wget >/dev/null 2>&1; then
+    if ! command_available 'wget'; then
 	echo >&2 "ERROR: an installed wget is required for a network-based update"
 	exit 1
     fi
@@ -472,11 +487,11 @@ create_gpg_home() {
 prepare_gpg_home() {
     eval $(apt-config shell GPG_EXE Apt::Key::gpgcommand)
 
-    if [ -n "$GPG_EXE" ] && command -v "$GPG_EXE" >/dev/null 2>&1; then
+    if [ -n "$GPG_EXE" ] && command_available "$GPG_EXE"; then
 	true
-    elif command -v gpg >/dev/null 2>&1; then
+    elif command_available 'gpg'; then
 	GPG_EXE="gpg"
-    elif command -v gpg2 >/dev/null 2>&1; then
+    elif command_available 'gpg2'; then
 	GPG_EXE="gpg2"
     else
 	echo >&2 "Error: gnupg or gnupg2 do not seem to be installed,"
@@ -569,9 +584,9 @@ case "$command" in
     verify)
 	GPGV=''
 	eval $(apt-config shell GPGV Apt::Key::gpgvcommand)
-	if [ -n "$GPGV" ] && command -v "$GPGV" >/dev/null 2>&1; then true;
-	elif command -v gpgv >/dev/null 2>&1; then GPGV='gpgv';
-	elif command -v gpgv2 >/dev/null 2>&1; then GPGV='gpgv2';
+	if [ -n "$GPGV" ] && command_available "$GPGV"; then true;
+	elif command_available 'gpgv'; then GPGV='gpgv';
+	elif command_available 'gpgv2'; then GPGV='gpgv2';
 	else
 	   echo >&2 'ERROR: gpgv or gpgv2 required for verification'
 	   exit 29

debian/apt.cron.daily

@@ -324,7 +324,7 @@ check_power(){
     #       1 (false)   System is not on main power
     #       255 (false) Power status could not be determined
     # Desktop systems always return 255 it seems
-    if command -v on_ac_power >/dev/null 2>&1; then
+    if which on_ac_power >/dev/null; then
         on_ac_power
         POWER=$?
         if [ $POWER -eq 1 ]; then
@@ -352,7 +352,7 @@ if test -r /var/lib/apt/extended_states; then
 fi
 
 # check apt-config existence
-if ! command -v apt-config >/dev/null 2>&1; then
+if ! which apt-config >/dev/null ; then
 	exit 0
 fi
 
@@ -388,7 +388,7 @@ fi
 check_power || exit 0
 
 # check if we can lock the cache and if the cache is clean
-if command -v apt-get >/dev/null 2>&1 && ! eval apt-get check $XAPTOPT $XSTDERR ; then
+if which apt-get >/dev/null && ! eval apt-get check $XAPTOPT $XSTDERR ; then
     debug_echo "error encountered in cron job with \"apt-get check\"."
     exit 0
 fi
@@ -451,7 +451,7 @@ UPDATE_STAMP=/var/lib/apt/periodic/update-stamp
 if check_stamp $UPDATE_STAMP $UpdateInterval; then
     if eval apt-get $XAPTOPT -y update $XSTDERR; then
 	debug_echo "download updated metadata (success)."
-	if command -v dbus-send >/dev/null 2>&1 && pidof dbus-daemon >/dev/null; then
+	if which dbus-send >/dev/null && pidof dbus-daemon >/dev/null; then
 	    if dbus-send --system / app.apt.dbus.updated boolean:true ; then
 		debug_echo "send dbus signal (success)"
 	    else
@@ -487,7 +487,7 @@ fi
 
 # auto upgrade all upgradeable packages
 UPGRADE_STAMP=/var/lib/apt/periodic/upgrade-stamp
-if command -v unattended-upgrade >/dev/null 2>&1 && check_stamp $UPGRADE_STAMP $UnattendedUpgradeInterval; then
+if which unattended-upgrade >/dev/null && check_stamp $UPGRADE_STAMP $UnattendedUpgradeInterval; then
     if unattended-upgrade $XUUPOPT; then
 	update_stamp $UPGRADE_STAMP
 	debug_echo "unattended-upgrade (success)"

debian/rules

@@ -24,7 +24,7 @@ confflags += --with-procs=$(PARALLEL)
 # See below
 -include build/environment.mak
 
-ifneq (,$(shell command -v dpkg-buildflags))
+ifneq (,$(shell which dpkg-buildflags))
   # make does not export to $(shell) so we need to workaround 
   # (http://savannah.gnu.org/bugs/?10593)
   dpkg_buildflags = DEB_BUILD_MAINT_OPTIONS=hardening=+all dpkg-buildflags